My opinion is this. I try to stay out of discussions that I know I have a propensity to get irrational in. Some people like to spar, and some people even like to spar with their friends. I would rather just have friends and ignore the obvious differences of opinion.
I think the surest way to loose control of one's self is to get involved in a conversation that tries to change somebody else's mind on Religion and Politics. I am willing to bet that it is here that most people that get banned have crossed a line.